Safe operation of a gas turbine engine dictates that the engine be periodically inspected. Accordingly, plugged openings are provided in the casing for borescopic insertion providing for inspection of interior components.
The plug must be secure during normal operation with a seal sufficiently snug to prevent leakage. Such a plug, however, tends to bind after engine operation thereby often requiring special pulling tools for plugs. Particularly with aircraft engines such operation may at times be carried out at locations where the special tools are not available.
The interior surface of the plug must be smooth and aligned with the interior casing surface to avoid airflow disturbances which would have a negative effect on the flow distribution entering a compressor or turbine. Threaded openings are expensive, they create stress concentrations, and they present great difficulties in properly aligning the interior surface with the surface of the casing.